Here is the scenario: I have an outdoor pond with a bio-falls and two small streams running into the pond itself. One of the streams is relatively slow moving, which I conclude is due to its width and depth. I’m contemplating trying to increase the speed at which the stream empties into the pond. Changing the width is not an easy task. Changing the depth would be easier (just add a bunch of gravel for example).
My question is: Would adding gravel to the stream actually increase the speed of the water flow? Or would the speed just stay the same because the level of the stream would rise to keep the depth constant?
The flow of the water into the stream from the bio-falls would remain constant. Also, the streams empty into the pond via small waterfalls.
I would think that by making the final stretch of the channel a little more shallow, you would increase the speed. Since you will always have x gal/min moving through the whole channel the speed of flow times the cross sectional area of the channel needs to remain constant. So if you make the end of the channel shallower, the speed of water should increase.
If you introduce a restriction into the flow, the velocity of the water flowing over the restriction will be faster than before. The flow upstream and downstream will be unchanged.
The level will be unchanged, unless you build up the stream bed to higher than the existing water surface - if you do, the upstream level will increase to a level very slightly higher than the level of the new dam.
If you put the restriction right at the discharge point (at the top of the waterfall) you will be able to achieve an increase in velocity at that point, causing the water to be projected further in the direction of flow before falling into the pond. This might be the effect you are trying to achieve.
Loose gravel is probably not the ideal material for this job - it will tend to get washed into the pond and will disrupt the flow. A smooth concrete “nozzle” or ramp would do a better job.